At surgery, a yellowish and firm lesion associated with abnormal vessels was resected.

The patient's recovery was good and two months later an isolated mild clumsiness of the right hand was noted. However the patient was able to resume his daily hobby of golfing. Total body PET scan using fluorodeoxyglucose was normal, as were motor and sensory nerve conduction studies and echocardiography. Serum beta2 microglobulin was within normal limits and bone marrow biopsy showed no abnormality. No change in the patient condition was noted over a follow-up period of more than one year.

MICROSCOPIC PATHOLOGY

Microscopic examination on haematoxylin and eosin stained sections from three specimens revealed amorphous eosinophilic material, either extra-vascular or thickening vessel walls (Figure 2a). In close proximity to the deposited material, the cerebellar parenchyma was infiltrated by predominant mature plasma-cells (CD 138 immunopositive; Figure 2b) associated with a few mature T lymphocytes (CD3 immunopositive; Figure 2c) and macrophages of the foreign body type (CD68 immunopositive; Figure 2d). The weak congophilia of the deposits failed to show any birefringence in polarized light microscopy. There was a strong immunostaining of the deposited material as well as the plasma-cells with anti-κ light chain (Figure 2e), while anti-λ antibodies failed to label them. The deposits were also immunonegative for transthyretin amyloid, Aβ amyloid and AA amyloid. Under UV light, the deposits were slightly fluorescent after thioflavine staining, strongly immunofluorescent for κ light chain (figure 2f) and negative for λ light chain. Electron microscopy revealed the non-fibrillar ultrastructure of the deposits which presented as finely granular aggregates (Figure 3).